1. Technical Field
The present invention relates to spindle motors and to disk drives furnished with the spindle motors.
2. Description of the Related Art
Spindle motors for rotatively driving recording disks such as magnetic disks chiefly comprise a rotor assembly that includes a shaft, and a stator assembly in which a sleeve is anchored. The shaft is disposed along the inner periphery of the sleeve, and a rotor hub that carries a recording disk(s) is fixed to the upper-end portion of the shaft. The sleeve is anchored by means such as an adhesive to a retaining device, typically a bracket portion, and via fluid-dynamic-pressure bearings rotatively supports the shaft.
Recording-disk drives employing spindle motors are furnished with magnetic heads for reading data from or writing data onto the recording disk(s).
A problem with the above-described spindle motors employed in recording-disk drives is static electricity. When a spindle motor is driven, spinning the recording disk(s) at high speed, friction between the disks and the enveloping air is produced, and due to the resulting static electricity the disks take on an electric charge. The disks taking on a charge ends up producing an electric-potential difference between the disks and the bracket portion. As a consequence the potential difference is applied across the disks and the magnetic heads, producing electrical discharge between the disks and the heads that could destroy the heads.
This risk is particularly pronounced with magnetoresistive (MR) heads as well giant magnetoresistive (GMR) heads, which due to a transition to large volume, high-density recording disks in recent years are being adopted as the magnetic heads in recording-disk drives. Because MR and GMR heads structurally include circuit elements having high electric-current densities, and because such heads are composed from thin films, not furnishing them with some sort of protective structure against electric potential differences puts the heads at risk of being damaged.
To guard against such problems, a configuration that establishes electrical continuity between motor components such as the rotor assembly and the stator assembly, electrically connecting the two, is necessary. To date the following mechanisms have been proposed for establishing such a continuity configuration.
For example, in a spindle-motor structure in which the rotor assembly is retained in the stator assembly via a fluid-dynamic-pressure bearing unit, continuity between the rotor and stator assemblies can be designed into the configuration by lending electroconductivity to the lubricating fluid. In another example, an electroconductive adhesive agent is used as the adhesive means for anchoring a sleeve and bracket portion forming a stator assembly, thus serving as a way to equalize the electric potentials of the sleeve and the bracket portion. And a spindle motor can be designed for electrical continuity between the sleeve and the bracket portion by plastically deforming a portion of the area of adherence between the sleeve and the bracket portion to conjoin them metallically.
Traditionally, it has usually been the case that the sleeve and the bracket portion are each formed from a different type of material. The thermal expansion coefficients of the sleeve and the bracket portion are therefore often different. Moreover, in implementations employing an electroconductive adhesive to anchor the sleeve and bracket portion, the thermal expansion coefficient of the adhesive is often different from that of the sleeve as well as that of the bracket portion.
When the rotor assembly of a spindle motor in such an implementation spins, vibrations and similar disturbances due to the rotation of the rotor assembly are transmitted to the sleeve via the fluid-dynamic-pressure bearing, and further are exerted on the electroconductive adhesive. Consequently, if it should happen that a sufficient amount of the electroconductive adhesive has not been applied between the sleeve and the bracket portion, there will be a danger that fissures and breaks in, and peeling off of, the adhesive due to vibrational and like disturbances will occur; such occurrences arising in the electroconductive adhesive impair the continuity between the sleeve and the bracket portion.
Meanwhile, because silver is the chief component in the majority of electroconductive adhesives, they are costly. Consequently, in most cases the amount of electroconductive adhesive used is made as slight as possible, which serves to curtail the expense; yet the continuity between the sleeve and the bracket portion will be impaired if there is too little electroconductive adhesive between them.